JSON

Custom mapping for the index you will update with NiFi flows

Unlike the Logstash output “ElasticSearch”, you cannot associate a customized mapping with the processor. Therefore, if the dynamic mapping of ElasticSearch doesn’t attribute the type you really want to one of your fields, you will have to use a default mapping template (see this chapter in the ElasticSearch section of the site).
If doing that, remember that:

NiFi and JSON

Remark: with the introduction of the records-oriented flow files, managing JSON with NiFi became easier than ever.
The below how-to about JSON manipulation is making an extensive use of message contents and attributes extraction / modification.
You will find later pages about the records usage in NiFi.